Welcome to France.
I've been living here since 1996 and all three of my children were born here. I'm American and also became French in 2002. I'm a former Flight Attendant and we fly a lot. We always take a car seat with us. As an ex-F/A, I've had to watch all those crash videos and yes, taking a car seat with you on the plane is much, much safer but more about that later.
Currently, I have my two older children in boosters. My 6 yo is in a hbb but she is off the charts for her age here. My 9 yo is in a lbb.
My youngest, age 4, is riding in a Radian which I brought from the States. Because we're on the German-French border, I looked in vain in both countries for a higher harnessed seat. Finally, I found out that the EU does NOT approve ANY higher harnessed seats. They test to 40lbs/18 kilos and then it's expected that the child ride in booster.
I wrote Sunshine Kids and asked them directly why they didn't sell in Europe and Russell wrote back that the Radian is approved to the max, 40lbs. but since they can't get a higher approval, they wont bother selling over here because the whole point of the Radian is to keep a child harnessed to 65 or 80lbs. If they aren't allowed to advertise this fact, it serves no purose to simply compete and yet-another 40lbs maximum seat.
I even had stores trying to locate higher harnessed seats of brands they carried and wrote Britax in France and asked why they didn't sell the Marathon, Regent, etc. All I got back was a poorly written (probably standard) letter saying that Britax takes children's safety seriously and totally missed the point. When I wrote back to clarify, I never had a response.
This is unacceptable and I brought the Radian over. A couple of years ago I was in a very small car accident but the police did arrive. I actually
installed the Radian into a French police car to go to the station. The guy was even joking about it being a French car, implying he realized that it was a foreign seat. He even wanted to save me the trouble and said I could ride with her with just the seatbelt. Heck no! I was just in an accident...
Which brings me to the point. Obviously I couldn't replace it or have my insurance do so. This accident occurred at a very low speed, in heavy traffic and the damage was far from the seat. The car was actually driveable EXCEPT for the fact that a piece of metal was sticking into the tire in front. I had to balance the fact that my dd is in a 5 point harness vs. the potiential issue with using a seat which has been in an accident. Given the minor nature of the accident, I continued to use the seat.
Technically, I'm breaking the law here in France. The police actually stop people quite regularily to check various papers and whether the children are strapped in. They have never checked whether the seat was legal. It actually is up to 40lbs. because it has "UN" approval but I'm unsure what that entails so I don't want to debate that.
I'm not sure how my insurance company would react if they knew about the seat.
Many, many children here are totally unrestrained. Not only are tiny 2 and 3 year olds expected to ride in boosters but it's legal to turn a child ffing at 9 months and 9 kilos. The quality of seats is far below what you find in N. America. Baby buckets still usually only have 3 point harnesses.
Tethers are rare but becoming more common. It's difficult to find cars with tether hooks. I bought a Toyota Yaris. Not all new Toyotas have them but most do. At a Swedish car dealership, they offered to retro-fit them.
Be aware that French roads and parking spaces are much, much narrower than in the U.S. If you will have two cars, get a big one and let your dh use it for work. You take the small one for the school run, etc. You want to be able to parallel park on a dime! You only have two children so that will open up your options. I had trouble finding a car with three shoulder belts in the back. Don't end up parking at the end of the lot because you can't squeeze your car into a closer spot. Don't make your children do unnessary walking because you have to have a bigger car. You simply don't need it. When we go on vacation, etc. my dh has the Landrover but I get by with my Yaris just great.
Make sure you're really comfortable with stick shift (I wasn't when I came-big mistake!) and be aware that the kind of driving here is very defensive. People do really crazy things, like left turns from the right lane on red lights, etc. Do NOT put on your blinkers unless you are going to change lanes right then. You look for a space and THEN turn on your signals (a habit I have to stop when I'm in California). Also, expect wear and tear to your tires at a higher rate than back home. Check them often, be aware that you'll need to change them more often and don't have it contribute to an accident the way my worn-down ones did to me last summer!
Try to avoid getting a French driver's license AT ALL COSTS because it costs
a lot! Seven years ago, I probably spent about 800 euros getting mine. It's also stress and complicated. You get one year to drive on a non-approved license. I know people who actually get approved licenses from one of the accepted states before moving to France. The State Dept. has the list (I can't pull it up right now).
About the air travel...
As you probably know, the Regent is
not FAA certified. It was one I also considered but opted for the Radian partly for this reason. While Britax obviously sells here, they don't carry that model, only those that harness to 40lbs/18 kilos like I stated earlier.
While air travel is safe, I can confirm that after flying for a living for 13 years, the chances of your checked-in car seat being lost and/or damaged are VERY high. I trotted off a transatlantic last winter to find a lonely one of our four checked bags had made it. Sure, they delivered them two days later but my dd was safe on the 2 hour drive home because we had her Radian with us. Thank goodness! Please, take the seat on board with you.
Which also brings up the case with approved car seats on airplanes... Before the Radian, I had a Sit-n-Stroll and a old version of the Scenera for flying (we do about two trips a year between Europe and California). We also fly in this region of the world.
Air France-was able to use my seat quite a few times without question.
Lufthansa-only once was my U.S. car seat removed. Usually I can fly with it, without question.
Iberia-seat taken away
Ryan Air-seat not allowed on board. Gate agent even called to ask for an exception. Car seats not allowed at all in the cabin.
Charter companies-can't get the info and usually not allowed. Pitch is often very, very tight on these flights. I didn't try because we weren't driving at our destination anyway.
I try to book on U.S. companies when possible, which it isn't always...
Here are my flying tips. Please note that this is a totally non-commercial article written originally for the local American club newsletter;
http://flyingwithchildren.blogspot.com
My contact info is at the end. I have enough advice for you to fill a truck, if you want it (books to read, health care, etc.)
Meanwhile, good luck with all the preparations. It's a great place to live but this is one of the issues you will encounter while living here.
Good luck!
Sharon in Alsace
There are some different options available to you. You will not be able to legally use US car seats in France. If you're military, different rules apply but I doubt this is the case since we're talking about France. You're allowed to use US car seat for a limited time if you;re a visitor, if you move to Europe you're legally required to use an approved European seat (ECE R44). 
